“The optimal spacing gap depends on when the information will be tested in the future. For participants who completed the final test 7 days after their final study session, the optimal spacing gap was 1 day. However, for participants who waited 35 days before taking the final test, the optimal spacing gap was 11 days. For those who completed the final test after 70 days, the best spacing gap was 21 days. In general, the optimal spacing gap equaled 10 – 20 % of the test delay. In other words, the longer the test delay, the longer the optimal spacing gap.”

Not great evidence for expanding (increasing intervals between reviews) vs fixed schedules

For teachers, one suggestion: the dreaded cumulative test, since it forces students to go back over the old material